This invention relates in general to a Micro Electro Mechanical Systems (MEMS) die. In particular, this invention relates to an improved method for attaching the MEMS die to a mounting surface that provides an attachment layer between the MEMS die and the mounting surface and that reduces the transmission of stress to the MEMS die.
According to a known method, solder paste or a solder preform is placed onto a mounting surface, such as a pedestal of a valve, fluid control device, fluid system parameter sensing device, and the like. A MEMS die is then placed onto the solder paste or the solder preform and the solder paste or the solder preform is heated in a re-flow operation.
During attachment of the MEMS die, such as a MEMS die configured as a MEMS fluid pressure sensor, to a mounting surface, mechanical stress may be generated and transmitted to the MEMS fluid pressure sensor. Such mechanical stress may adversely affect the MEMS fluid pressure sensor's performance.
For example, the MEMS fluid pressure sensor may use a Wheatstone bridge strain gauge. Such a MEMS fluid pressure sensor may have a chamber with a flexible wall that deforms in response to fluid pressure in the chamber, thus producing strain. In this context, strain is the response of a system to an applied stress. When a material is loaded with a force, it produces stress, which may then cause the material to deform. As used herein, engineering strain is defined as the amount of deformation in the direction of the applied force divided by the initial length of the material. The strain gauge senses this deformation, and generates an output signal representative of the fluid pressure in the pressure chamber.
In one conventional device, the MEMS fluid pressure sensor is soldered to the mounting surface of the device body or base, and the base then threaded into a housing, such as a Schrader valve housing. As the base is tightened into the housing, applied torque may generate non-transitory stress in the base that is transmitted through the solder to the MEMS fluid pressure sensor, such that the strain gauge detects a strain and will erroneously report a fluid pressure when no fluid pressure is present in a pressure chamber of the MEMS fluid pressure sensor.
Thus, it would be desirable to provide an improved method for attaching a MEMS die to a mounting surface that reduces the transmission of stress to the MEMS die.